Node.js 可读流 _read 用法

Question

我了解如何在 Node 的新 Streams2 库中使用可写流，但我不了解如何使用可读流。

例如，围绕 dgram 模块的流包装器:​​

var dgram = require('dgram');

var thumbs = {
  twiddle: function() {}
};

var defaults = {
  address: '0.0.0.0',
  type: 'udp4',
  port: 12345,
  broadcast: null,
  multicast: null,
  multicastTTL: 1
};

var UDPStream = function(options) {
  if (!(this instanceof UDPStream))
    return new UDPStream(options);

  Duplex.call(this);

  options = options || {};

  this.address = options.address || defaults.address;
  this.type = options.type || defaults.type;
  this.port = options.port || defaults.port;
  this.broadcast = options.broadcast || defaults.broadcast;
  this.multicast = options.multicast || defaults.multicast;
  this.multicastTTL = options.multicastTTL || defaults.multicastTTL;

  this._socket = dgram.createSocket(this.type, setup.bind(this));
  this._socket.on('message', this.push.bind(this));
};

util.inherits(UDPStream, Duplex);

var setup = function() {
  if (this.multicast) {
    this._socket.addMembership(this.multicast);
    this._socket.setMulticastTTL(this.multicastTTL);

    this.destination = this.multicast;
  } else {
    // default to using broadcast if multicast address is not specified.
    this._socket.setBroadcast(true);

    // TODO: get the default broadcast address from os.networkInterfaces() (not currently returned)
    this.destination = this.broadcast || '255.255.255.255';
  }
};

UDPStream.prototype._read = function(size) {
  thumbs.twiddle();
};

UDPStream.prototype._write = function(chunk, encoding, callback) {
  this._socket.send(chunk, 0, chunk.length, this.port, this.destination);
  callback();
};

module.exports = UDPStream;

除了 _read 实现之外，一切都有意义。这简直就是在胡闹，因为我不明白我应该在那里做什么。当 udp 套接字发出新消息时，我的数据被推送，但我无法暂停或恢复底层资源。这应该是什么样的？

Answer 1

_read 是暂停恢复机制的一部分。来自 NodeJS API 文档

When data is available, put it into the read queue by calling readable.push(chunk). If push returns false, then you should stop reading. When _read is called again, you should start pushing more data.

因此，在您的 _write 函数中，如果 socket.send 调用因返回 false 或调用带有错误的回调而失败，您应该暂停流。 _read 然后可以简单地做 this._paused = false

可能看起来像这样。

UDPStream.prototype._read = function() {
  this._paused = false;
}

UDPStream.prototype._write = function(chunk, encoding, callback) {
  if(!this._paused)
   this._socket.send(chunk, 0, chunk.length, this.port, this.destination);
};